The present invention comprises an improvement on the invention described and claimed in copending Gonzalez U.S. Patent Application Ser. No. 214,904, filed Dec. 10, 1980, now U.S. Pat. No. 4,377,892, issued Mar. 29, 1983, for "Method of Fabricating Sintered Metal/Polymer Impregnated Ball Valve Seats", assigned to the assignee of the instant application.
The aforementioned Gonzalez patent, the disclosure of which is incorporated herein by reference, describes a sintered metal seat fabricated of a compacted mass of particles of a corrosion resistant metallic alloy, e.g., stainless steel or bronze, whose pores or interparticulate spaces are impregnated with a polymeric material such as PTFE or TFE, describes the method of fabricating such seats, and discusses the advantages which such seats exhibit in valve applications intended to operate at comparatively high pressures. The seat is prepared by initially fabricating a sintered metal "green compact" structure in the form and shape conventionally employed for ball valve seats, thereafter sintering the green compact to fuse adjacent metal particles to each other, then impregnating the resultant seat with an emulsion of uncured polymeric material having lubricity, the impregnation being effected by means of a vacuum and/or positive pressure step, and then drying the resultant polymer impregnated seat whereafter the seat is placed in a furnace to sinter and cure the polymer which remains in the interparticulate spaces of the seat. Following these steps, the polymer impregnated seat is subjected to a coining step, i.e., extremely high pressures are applied to the exterior of the seat to collapse substantially all of the interparticulate cavities and voids throughout the seat onto the cured polymer which is enclosed within the seat, to eliminate all interparticulate voids in the sintered metal seat to the extent possible and to render the complete seat impervious to fluid flow, i.e., to make the final product "leak free" throughout.
Seats constructed in accordance with the foregoing procedure exhibit substantially uniform density throughout the body of the seat and at the exterior surfaces of the seat, and have been found in practice to hold bubble tight on helium, which is the most stringent leakage test available. The seats have a limited temperature capability, however, and it has been found that when seats of the Gonzalez type are operated at temperatures above 650.degree. F. (the jell point of TFE), the TFE or other polymers impregnated into the seat begin to break down. Some of the polymer material vaporizes at these comparatively high temperatures, but other portions of the polymer simply extrude from the seat and adhere to and contaminate the ball of the valve with which the seat is associated.
There is need for a valve capable of operating at, for example, 1000.degree. F. Such valves are commercially available at the present time, and typically employ seats fabricated of "Graphitar" (Trademark) produced by Wickes Engineered Materials Corp. This seat material is a slightly porous pure graphite sintered structure. Its major disadvantage is its brittleness, i.e., under severe thermal changes or shock loadings, the seat material fractures and the valve fails.
It is the object of the present invention to provide a seat which avoids the foregoing problems of the Gonzalez and Wickes seats and which, more particularly, can operate at temperatures up to 1000.degree. F. and pressures up to 1000 psi by combining the high strength of the sintered metal matrix employed in the Gonzalez seat with the high temperature capability, anti-galling characteristics and good chemical inertness of graphite, to produce a valve seat having superior high temperature/high pressure capabilities.